Psoriasis is a common human skin disease that affects approximately 2% of the population including an estimated 400,000 veterans. In addition to activation of the immune system and inflammation, psoriasis is characterized by hyperproliferation and abnormal differentiation of epidermal keratinocytes of the skin. Previous data from our laboratory indicate that in epidermal keratinocytes the lipid-metabolizing enzyme phospholipase D2 (PLD2) and the glycerol transporter aquaporin-3 (AQP3) physically and functionally interact. AQP3 transports glycerol into the cell, making it available to the associated PLD2, which can then use this glycerol to synthesize phosphatidylglycerol (PG). Our results further demonstrate that PG acts as a lipid second messenger to inhibit keratinocyte proliferation, stimulate differentiation and inhibit inflammation. The importance of AQP3 in skin is emphasized by the fact that mice lacking the gene for AQP3 exhibit an epidermal phenotype characterized in part by decreased epidermal glycerol content and delayed barrier repair. The phenotype can be corrected by pharmacologic doses of glycerol but not other humectants, consistent with the fact that glycerol only inefficienty enters cells in the absence of aquaglyceroporins like AQP3. In addition, AQP3 has been found to be dysregulated in human skin diseases. Thus, for example, we have previously reported that AQP3 protein levels are decreased and diffuse in psoriasis; however, other investigators have found an up-regulation of AQP3 mRNA expression in psoriatic lesions. We first propose to resolve whether AQP3 protein levels (compared with mRNA levels) are increased or decreased in psoriasis using western and qRT-PCR analysis of psoriatic and healthy epidermis. In addition, we will measure PLD2 and glycerol levels in these samples, with the idea that reductions in one or both of these parameters could also lead to decreased levels of anti-proliferative, pro-differentiative PG. In these patients we will also investigate the effect of glycerol administratio on psoriasis with the expectation that this agent will improve psoriatic lesions. Despite the importance of AQP3 to skin function, there is little information available concerning the mechanisms regulating AQP3 expression in health or disease. In initial results we have found that AQP3 mRNA and protein levels are increased by treatment with a broad-spectrum histone deacetylase (HDAC) inhibitor, suggesting that basally one or more HDACs normally represses AQP3 expression. Additional studies with a panel of HDAC inhibitors and overexpression and knockdown of HDAC3 (versus HDAC1) indicate that this repressive HDAC is HDAC3. In the proposed studies we will explore the mechanism by which HDAC3 regulates AQP3 expression, testing the hypothesis that HDAC3 normally suppresses AQP3 expression by decreasing the acetylation and transcriptional activity of one or more members of the p53 family of transcription factors, in addition to its effects on histones and chromatin organization. We will examine this idea in vitro and in an epidermal-specific conditional HDAC3 knockout mouse model in vivo, monitoring the mRNA and protein levels of AQP3 in comparison with floxed HDAC3 control littermates. We will also examine the effect of keratinocyte-specific HDAC3 deletion on epidermal structure and function and the development of psoriasiform lesions in the imiquimod mouse model of psoriasis, with and without co-treatment with glycerol. Completion of the proposed research should result in an understanding of the role of HDAC3 in regulating AQP3 expression as well as a potential proof-of-principle for HDAC3 inhibition, with or without concomitant glycerol administration, as a therapy for psoriasis.